Field of the Invention
The present invention relates to a detection apparatus which detects a mark, formed on the lower surface of a target object, from the upper surface side of the target object, an exposure apparatus, and a method of manufacturing a device.
Description of the Related Art
To manufacture a device (for example, a semiconductor device, a liquid crystal display device, or a thin film magnetic head) using the photolithography technique, a projection exposure apparatus which projects and transfers a pattern drawn on a reticle onto, for example, a wafer by a projection optical system has been conventionally employed. At this time, an alignment detection system built into the projection exposure apparatus is used to align an image of the pattern of the reticle, which is projected via the projection optical system, with the pattern already formed on the wafer, and exposure is performed.
With miniaturization and an increase in packing density of integrated circuits, a projection exposure apparatus is required to project and transfer the pattern of a reticle onto a wafer by exposure at a higher resolution. The minimum line width (resolution) that the projection exposure apparatus can transfer is proportional to the wavelength of light used for exposure, and is inversely proportional to the numerical aperture (NA) of a projection optical system. This means that the shorter the wavelength, the higher the resolution. Hence, the recent light sources have shifted from the g-line (wavelength: about 436 nm) and i-line (wavelength: about 365 nm) of ultra-high pressure mercury lamps to a KrF excimer laser (wavelength: about 248 nm) and an ArF excimer laser (wavelength: about 193 nm). Also, the practical application of an F2 laser (wavelength: about 157 nm) as a light source is in progress, so the adoption of EUV (Extreme Ultra Violet) light having wavelengths of several to one hundred nanometers is expected in the future.
The exposure apparatus has come to be used to manufacture special devices including not only the conventional IC chips such as memory and logic chips but also stacked devices, which use a through-hole via process, such as a MEMS and a CMOS image sensor (CIS). Devices such as a MEMS and a CIS are different from IC chips in several respects. In devices such as a MEMS and a CIS, demands for the line width resolution and overlay accuracy of IC chips are easy, while a large depth of focus is necessary. Also, as special processes for manufacturing devices such as a MEMS and a CIS, a process of setting an alignment target on the lower surface of an Si wafer, and exposing the upper surface of the wafer to light upon alignment with this lower surface is available. As a typical practical example, the thickness of an Si wafer is reduced, and a through-hole via is formed from the upper surface side and electrically connected to the circuit on the lower surface. A technique of detecting an alignment mark formed on the lower surface (lower surface alignment) in this way has become important these days.
Japanese Patent Laid-Open No. 2002-280299 proposes a method of using an alignment detection system set on the lower surface side (wafer chuck side) to form, on the upper surface, an image of an alignment mark formed on the lower surface, and detect the position of the alignment mark on the upper surface. However, in a method of setting an alignment detection system on the lower surface side in this way, a hole is formed at a specific position for the wafer chuck, so only the alignment mark at this position can be measured. Therefore, in the method disclosed in Japanese Patent Laid-Open No. 2002-280299, it is impossible to observe an alignment mark positioned at an arbitrary position on the lower surface of the wafer.
An Si substrate is transparent to infrared light (wavelength: 1,000 nm or more). Hence, a method of observing a mark on the lower surface from the upper surface side using a position detection system that uses infrared light as a light source has also been proposed. In a normal alignment sequence, first, to measure a best focus position of an alignment mark, an image of the alignment mark is obtained while a wafer stage is driven in the optical axis direction of a position detection system, and a position with a maximum contrast is calculated. This measurement method will be referred to as image autofocus measurement hereinafter. High-accuracy position detection can be performed by alignment at a focus position calculated by image autofocus measurement.
In image autofocus measurement, an image of an alignment mark is obtained by driving a wafer stage from the default focus position of a position detection system in the Z-direction. The default focus position of the position detection system is aligned with a reference plate positioned on a stage in an exposure apparatus. That is, in the conventional image autofocus measurement, a measurement start point is determined with reference to the level of the reference plate. However, the level of the upper surface of the wafer is often different from that of the reference plate, depending on the degree of suction of a wafer in mounting the wafer on the wafer stage. In this case, it is impossible to start image autofocus measurement for an alignment mark on the upper surface of an Si substrate, precisely from the upper surface of the Si substrate. However, when image autofocus measurement starts from the level of the reference plate, it is possible to quickly, easily detect the mark on the upper surface of the Si substrate.
However, when an alignment mark is present on the lower surface of the Si substrate, it is normally problematic in that the position detection system focuses on the reference plate. When the wafer stage is driven from the reference plate at the default focus position of the position detection system, a large search range is necessary to detect the alignment mark on the lower surface of the Si substrate. When a large search range is set to measure the alignment mark on the lower surface of the Si substrate, the measurement operation takes a considerable time, thus lowering the throughput. Also, the calculation error of a best focus position of the alignment mark also increases as the measurement pitch of image autofocus measurement increases, thus making it impossible to perform high-accuracy alignment.